1. Field of the Invention
This invention is concerned with a novel hull construction for a liquid-cargo tanker ship. The hull construction provides means for self-rescue of cargo while maintaining floatation, proper load distribution, stability and trim in case of grounding or collision.
2. Discussion of Related Art
Tanker ships in general usually are built with the main cargo tanks arranged along each side of the centerline of the ship. Ballast tanks interspersed with additional cargo tanks form wing tanks outboard of the main cargo tanks. When the ship is fully loaded with cargo, the ballast tanks are normally empty because, if for no other reason, it is now illegal to carry cargo in a ballast tank. Thus, in the event of collision or grounding of a fully-loaded ship, one or more of the empty ballast tanks and/or the outboard cargo wing tanks take the brunt of the impact. Presumably with that arrangement, the centrally-located cargo tanks remain relatively undamaged and rapid cargo leakage is minimized.
What actually happens however is that a punctured external ballast tank(s), or for that matter the void in a double-hulled ship, is flooded with sea water, the unbalanced weight of which causes the ship to list and/or to capsize due to the loading imbalance.
Since today's tankers have no emergency transfer system, they must wait hours or even days for delivery of pumps and containment devices to regain trim, balance, load redistribution and floatation. That wait may prove to be catastrophic.
An Exxon Background Series paper entitled Reducing Tanker Accidents (1973), in a discussion of double-bottom ship grounding, stated at page 16 that ". . . This loss of buoyancy complicates freeing a stranded ship and increases the probability of suffering total loss . . . "
In the matter of conventionally-located segregated ballast tanks (SBT), at page 43 of TANKER SPILLS, PREVENTION BY DESIGN, published by the National Research Council, it is stated that ". . . For a given cargo volume, the ballast volume increases a great deal in SBT space--in the range of 234 to 334 percent--which is indicative of the additional area that must be protected from corrosion. The expected oil outflow in grounding increases by up to 90 percent in many SBT designs . . . "
Further at page 89 of the same volume, we read ". . . Chevron Shipping Company has demonstrated . . . that the damage stability of an `industry standard` 130,000 DWT tanker greatly exceeds present [safety] criteria . . . A comparable double hull tanker, with interhull spacing of 2 meters and fully loaded, also easily exceeds criteria but can withstand less damage than the single hull design. When 96 percent loaded with heavy crude, the 2-meter double hull ship will capsize with only three tanks flooded . . . "
An NTIS report to Congress PB93-128874, page 17, illustrates the hull design of a conventional single-hull ship with outboard wing ballast tanks. When sailing filled with cargo, the ballast tanks of the tanker are empty. If a ballast tank on one side of the ship is damaged and becomes water-flooded, the effect on the ship's lateral stability of the loading imbalance is evident from the drawing.
U.S. Pat. No. 4,960,347 issued Oct. 2, 1990 to Booth B. Strange and assigned to the assignee of this invention, provides a normally empty, emergency holding tank and a cargo transfer system for moving liquid cargo from a damaged cargo tank to the holding tank. The placement of the holding tank is such that the ship's stability is not seriously affected. The liquid cargo that is removed from the damaged tank is replaced to some extent by seawater; a water seal is quickly established in the damaged region so that the damaged tank need be only partially emptied. Therefore, the total ship's displacement is not significantly altered. The teachings of the '347 patent are incorporated herein by reference.
U.S. Pat. No. 4,389,959 issued Jun. 28, 1983 to C. S. Conway teaches a system for removing liquid cargo from a damaged tank to create a water seal over the leak although he does not provide positive means for disposal of whatever liquid cargo is removed.
The patents cited assume a single-hulled tanker but one that has no outboard ballast tanks such as exist in the real world. Those references failed to consider the effects on ship's trim, in the presence of one or more flooded outboard ballast tanks.
The Federal Register for Friday, Oct. 22, 1993 mentions an as-yet untried Emergency Rapid Transfer System (ERTS). According to the text at page 54879, the ERTS consists of pipes and blank flanges that connect cargo tanks to ballast tanks. When damage occurs and the cargo tank level drops, sensors automatically cause the flange bolts to be ruptured. Cargo flows rapidly from the damaged cargo tanks into the empty ballast tank by force of gravity.
There are distinct disadvantages to the ERTS system in that the oil flow is uncontrolled and apparently can not be shut off when and if necessary. The presence of explosive bolts creates a fire hazard
In the presence of an accident, it is important to know the location of a damaged tank(s). U.S. Pat. No. 5,018,113 issued May 21, 1991 to Booth B. Strange et al. and assigned to the assignee of this invention teaches a method for locating a damaged tank using acoustic means.
U.S. Pat. No. 3,745,960, issued Jul. 17, 1973 to W. B. Devine teaches the concept of locating the ballast tanks along the centerline of a tanker. The ballast tanks are interspersed with cargo tanks and may be used for both cargo and ballast at the user's option.
In the above-cited NTIS report, page 19, there is shown the "POLMIS" tanker. External void spaces are provided outboard of the cargo tanks. A bladder-type ballast bag occupies a portion of a centrally-located cargo tank. Damage to, and subsequent water flooding of the void spaces could be disastrous.
For purposes of brevity but not by way of limitation, the terms "liquid cargo", "fluid cargo" or other similar phrases may be replaced by the word "oil".
There is a need for an improved hull design for an oil tanker ship that, if damaged, retains the following capabilities:
1. No significant loss of floatation;
2. No substantial disruption of the optimal load distribution;
3. No loss of trim due to unbalanced loading; and
4. No flow of oil into confined areas such as a ruptured ballast tank or the inter-hull voids in a double-hulled ship,
It is an object of this invention to provide a tanker-ship construction design that consists of a row of interconnected protected, inerted ballast tanks along the centerline of a tanker ship, flanked on each side by sets of cargo tanks.